CN101769722A - Method for heterodyne temporal series speckle interferometry of object deformation - Google Patents

Abstract

The invention discloses a method for measuring object deformation by heterodyne time series speckle interferometry, which belongs to the field of laser speckle interferometry. This method combines time series speckle interferometry and heterodyne interferometry, directly gives the phase function of the time-varying field, and introduces frequency difference to ensure the accuracy of deformation measurement. The invention simplifies the object deformation detection system, and better adapts to the storage speed of the high-speed CCD by using the lithium niobate polarization controller.

Description

A Method of Heterodyne Time Series Speckle Interferometry to Measure Object Deformation

technical field

The invention relates to the field of laser speckle interferometry, in particular to a method for measuring object deformation by heterodyne time series speckle interferometry.

Background technique

When the laser is irradiated on the surface of an object with diffuse reflection properties, according to the Huygens principle, every point on the surface of the object can be regarded as a point light source, and the light reflected from the surface of the object will coherently superimpose in space, and interference will occur in the entire space. The formation of randomly distributed bright and dark spots is called laser speckle. The speckle phenomenon generally exists in the optical imaging process and changes with the deformation or movement of the object. Initially, people mainly studied how to reduce the impact of speckle. However, during the research process, it was found that speckle has certain rules before and after object deformation, and carries information about object surface deformation or displacement, and gradually produced many applications. In the early 1970s, the laser speckle interferometry method was developed. In addition to a series of advantages such as non-contact, remote sensing, intuitive, and full-field situation of the holographic interferometry method, it also has a simple optical path. The requirements for the surface of the specimen are not high, the requirements for the experimental conditions are low, and the calculation is convenient. Laser speckle interferometry technology has a wide range of uses. In addition to measuring the displacement and strain of objects, it can also be used in non-destructive testing, surface roughness measurement, and vibration measurement.

Electron speckle interferometry (ESPI) is a modern optical metrology method that has been widely used in monitoring the displacement or deformation of the diffuser surface at the beginning of this century. In previous metrology, they mainly used the form of intensity-related fringes to represent the displacement or deformation to be measured, but due to the complex environmental influences and the complexity and randomness of the fringes themselves, it is difficult to form a unified fringe processing Models and Quantitative Interpretation Methods. In order to solve the quantitative detection problem in speckle metrology, phase shift technology (PSESPI) is introduced in ESPI technology. This method can conveniently detect the deformation field quantitatively, but it needs multiple interferograms in the same deformation state for phase calculation. At the same time, the introduction of the phase shifting device also increases the complexity of the experimental system and the sensitivity to environmental noise, so PSESPI is not suitable for deformation detection in time-varying environments. Carrier Electron Speckle Interferometry (CESPI) introduces carrier fringes into the ESPI device, and using Fourier transform technology, the full-field phase can be demodulated, so it can be used in situations where PSESPI cannot be used, such as continuous deformation or sports fields, etc. .

From the traditional speckle method to ESPI, PSESPI, and CESPI, the measurement of the displacement field is based on two states, that is, the state before and after deformation, and no time parameter is involved. With the emergence of large-capacity computers, high-speed CCD and other equipment, it has become a reality to record digital sequence images within a certain time interval. In the late 1990s, C.Joenathan et al. in Germany proposed a new technique for measuring object deformation, called time-series speckle interferometry (TSPI). TSPI records the entire deformation process of the entire object, and then uses Fourier transform technology to extract the deformation information of the object, and observes the change of each point on the object over time. But there is a problem that the measurement accuracy is not high; once the deformation of individual points on the object to be measured is less than 5λ, it is difficult for TSPI to accurately measure the deformation of these points. In order to solve this problem, at the beginning of this century, C.Joenathan and others proposed a method of introducing a constant frequency difference, that is, heterodyne interferometry. The method used by C.Joenathan to generate frequency difference is: after the linearly polarized light emitted by the laser is spatially filtered, it is incident on a frequency shifter composed of a 1/2 wave plate and a 1/4 wave plate, in which the 1/2 wave plate Through external mechanical braking, it rotates at a certain angular frequency, and the 1/4 wave plate is fixed. When the linearly polarized light passes through the rotating 1/2 wave plate and the fixed 1/4 wave plate, it will become orthogonal linearly polarized light with two frequencies, and the two beams of light have a certain frequency difference. However, this method also has certain disadvantages: 1. Rotating 1/2 wave plate belongs to mechanical rotation, the stability is not high, and the vibration generated by mechanical rotation has a certain impact on the entire optical system; 2. When rotating the wave plate, it will produce A certain deflection makes it impossible for the plane where the 1/2 wave plate is located to be perpendicular to the incident light during the rotation, which will have a certain impact on the phase during the measurement process; 3. The method used by C.Joenathan is a laser The outgoing light first passes through the spatial filter and then the frequency shifter. Since the laser beam passes through the spatial filter, the beam will expand, while the 1/2 wave plate and 1/4 wave plate have a small aperture due to the limitation of manufacturing technology. The expanded laser beam is blocked, so that the measurement area is reduced.

Contents of the invention

The purpose of the present invention is to propose a heterodyne time-series speckle interferometry method for object deformation in view of the problems existing in the heterodyne interferometry technology described in the background art.

It is characterized in that, comprising the following steps:

Step 1: Select a He-Ne laser with an output frequency of linearly polarized light at ω;

Step 2: The linearly polarized light becomes orthogonal linearly polarized light with two frequencies after passing through the polarization controller;

Step 3: the above-mentioned orthogonal linearly polarized light containing two frequencies is spatially filtered and beam expanded through a spatial filter;

Step 4: After the beam expansion, the orthogonal linearly polarized light containing two frequencies is transmitted through the ordinary beam splitter, enters the polarization beam splitter prism and is split into two beams;

One of the beams of light is reflected by the polarizing beam splitter and passes upward through the incident plane mirror of the analyzer. After being reflected, it passes through the analyzer again, and is reflected at the polarizing beam splitter to the common beam splitter as a reference light;

The other beam of light is transmitted through the polarizing beam splitter and then incident on the object to be measured, where it is scattered, and the scattered light with the deformation information of the object is reflected back to the polarizing beam splitter and passes through it to enter the ordinary beam splitter as the measurement light;

Step 5: The measuring light and the reference light are reflected by the ordinary beam splitter, and interfere after passing through the analyzer to form a deformed speckle pattern of the object; the speckle pattern is imaged on a high-speed CCD;

Step 6: Perform Fourier transform on the speckle image obtained by the high-speed CCD to obtain a modulation frequency image and a spectrum image, wherein the deformation direction of each part of the object is determined through the modulation frequency image; the time-based deformation of the object is obtained through the frequency spectrum image.

The polarization controller is a lithium niobate polarization controller.

The present invention combines the advantages of time-series speckle interferometry and heterodyne interferometry, simplifies the detection system, and greatly improves the accuracy of measuring the deformation of the entire object; it better adapts to The storage speed of high-speed CCD is improved.

Description of drawings

Figure 1: Optical path diagram of the heterodyne time-series speckle interferometry method for measuring object deformation.

Detailed ways

The preferred embodiments will be described in detail below in conjunction with the accompanying drawings. It should be emphasized that the following description is only exemplary and not intended to limit the scope of the invention and its application.

As shown in Figure 1: the light source is a He-Ne laser 1, and the output frequency is linearly polarized light of ω, which is coupled into the lithium niobate polarization controller 2, and the polarization controller obtains orthogonal lines with two frequencies by applying an external voltage. The light is polarized, and the heterodyne modulation frequency is changed by changing the voltage. The Jones vector for orthogonal linearly polarized light with two frequencies is:

$\stackrel{<span\; class="notranslate">\; \&OverBar;</span>}{\mathrm{<span\; class="notranslate">\; E.</span>}}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; A</span>}}{\sqrt{\mathrm{<span\; class="notranslate">\; 2</span>}}}\left(\begin{array}{c}\mathrm{<span\; class="notranslate">\; expi</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; \&omega;</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; 2</span>}{\mathrm{<span\; class="notranslate">\; \&omega;</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}<span\; class="notranslate">\; )</span>\mathrm{<span\; class="notranslate">\; t</span>}\\ \mathrm{<span\; class="notranslate">\; expi</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; \&omega;</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 2</span>}{\mathrm{<span\; class="notranslate">\; \&omega;</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}<span\; class="notranslate">\; )</span>\mathrm{<span\; class="notranslate">\; t</span>}\end{array}\right)$

where A is the amplitude. The orthogonal linearly polarized light with two frequencies passes through the spatial filter 3 for spatial filtering and beam expansion, and then transmits through the ordinary beam splitter 4 and enters the polarization beam splitter prism 10 . At the polarization beamsplitter prism, the orthogonal linearly polarized light containing two frequencies is separated, and a beam of light is reflected by the polarization beamsplitter prism and passes through the analyzer 11 and enters the plane mirror 12, and passes through the analyzer 11 again after being reflected. The polarized beam splitter is reflected back to the ordinary beam splitter, which is called the reference light; after the other beam of light is transmitted through the polarized beam splitter, it is incident on the object to be measured—the iron plate 13, where it is scattered, and the scattering with the deformation information of the object The light is reflected back to the polarizing beam splitter and passes through it to enter the common beam splitter, which is called measuring light.

The measurement light and the reference light are reflected by the common beam splitter, and interfere after passing through the analyzer 5 to form a speckle pattern. The telecentric imaging system is composed of the first convex lens 6, the gap 7 and the second convex lens 8. The function is to image the speckle pattern on the high-speed CCD 9. The high-speed CCD can record the deformation process of the iron plate. The speckle pattern intensity function can be Expressed as:

I(x, y, t)=I ₀ (x, y){1+Vcos[Φ ₀ (x, y)+4ω ₀ t]}

Among them, I ₀ (x, y) is the average intensity of the interference field, V is the visibility of the contrast modulation, Φ ₀ (x, y) is the random phase, and 4ω ₀ is the previously generated heterodyne modulation frequency, which is used to modulate the speckle intensity. When the iron plate to be tested is not deformed, the heterodyne modulation frequency 4ω ₀ can be observed on the high-speed CCD. When the iron plate is deformed, the above formula becomes:

$\mathrm{<span\; class="notranslate">\; I</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; x</span>}<span\; class="notranslate">\; ,</span>\mathrm{<span\; class="notranslate">\; the\; y</span>}<span\; class="notranslate">\; ,</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>{\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; x</span>}<span\; class="notranslate">\; ,</span>\mathrm{<span\; class="notranslate">\; the\; y</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; \{</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; V</span>}\mathrm{<span\; class="notranslate">\; cos</span>}<span\; class="notranslate">\; [</span>{\mathrm{<span\; class="notranslate">\; \&Phi;</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; x</span>}<span\; class="notranslate">\; ,</span>\mathrm{<span\; class="notranslate">\; the\; y</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; 4</span>}{\mathrm{<span\; class="notranslate">\; \&omega;</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; +</span>\frac{\mathrm{<span\; class="notranslate">\; 4</span>}\mathrm{<span\; class="notranslate">\; \&pi;\&Delta;z</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; x</span>}<span\; class="notranslate">\; ,</span>\mathrm{<span\; class="notranslate">\; the\; y</span>}<span\; class="notranslate">\; ,</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; )</span>}{\mathrm{<span\; class="notranslate">\; \&lambda;</span>}}<span\; class="notranslate">\; ]</span><span\; class="notranslate">\; \}</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; )</span>$

Among them, Δz(x, y, t) is the deformation function of the iron plate. When the deformation of the iron plate approaches the polarization beam splitter, the value is positive; when the deformation of the iron plate is away from the polarization beam splitter, the value is negative.

Time series speckle interferometry is to use the Fourier transform method to extract the deformation function of the iron plate, and then observe the deformation of the iron plate intuitively.

The median modulation frequency generated by the iron plate deformation function Δz within a certain time t is:

${\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; med</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; x</span>}<span\; class="notranslate">\; ,</span>\mathrm{<span\; class="notranslate">\; the\; y</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; \&Delta;z</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; x</span>}<span\; class="notranslate">\; ,</span>\mathrm{<span\; class="notranslate">\; the\; y</span>}<span\; class="notranslate">\; ,</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; )</span>}{\mathrm{<span\; class="notranslate">\; \&lambda;t</span>}}$

The deformation produced by different points on the iron plate during the deformation corresponds to different median modulation frequencies, and its value will be added or subtracted from the heterodyne modulation frequency, depending on the direction of the deformation of the point.

After the continuous movement of the iron plate, the time-modulated change of the speckle intensity is caused. In this process, a large number of speckle patterns are collected by the high-speed CCD. Perform Fourier transform on the object deformation speckle intensity map obtained by the high-speed CCD, and process the data to obtain the modulation frequency map and spectrum map. The direction of deformation of each part of the object to be measured can be obtained from the magnitude of the obtained modulation frequency. An increase in the modulation frequency indicates that the point is moving towards the polarization beam splitter; on the contrary, a decrease in the modulation frequency indicates that the point is moving away from the polarization beam splitter. For a spectrogram, it is of the form:

F{I(x,y)}=A(f y)+Q(f+f ₀ y)+Q ^* (ff ₀ y) (2)

The above formula is the Fourier transform of formula (1), and it is filtered to obtain the positive or negative first-order component. Here, the positive first-order component Q(f+f ₀ ·y) is selected. Then perform an inverse Fourier transform on it:

F ^-1 {Q}=q(x,y)

Among them, the phase function Φ(x, y) is included in q(x, y), and:

Φ(x, y)=tan ^-1 {Im[q(x, y)]/Re[q(x, y)]} (3)

Among them, Re[q(x, y)] is the real part of q(x, y), and Im[q(x, y)] is the imaginary part of q(x, y), which can be obtained by formula (3) Phase function:

$\mathrm{<span\; class="notranslate">\; \&Phi;</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; x</span>}<span\; class="notranslate">\; ,</span>\mathrm{<span\; class="notranslate">\; the\; y</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>{\mathrm{<span\; class="notranslate">\; \&Phi;</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; x</span>}<span\; class="notranslate">\; ,</span>\mathrm{<span\; class="notranslate">\; the\; y</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; 4</span>}{\mathrm{<span\; class="notranslate">\; \&omega;</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; +</span>\frac{\mathrm{<span\; class="notranslate">\; 4</span>}\mathrm{<span\; class="notranslate">\; \&pi;\&Delta;z</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; x</span>}<span\; class="notranslate">\; ,</span>\mathrm{<span\; class="notranslate">\; the\; y</span>}<span\; class="notranslate">\; ,</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; )</span>}{\mathrm{<span\; class="notranslate">\; \&lambda;</span>}}$

Finally, the obtained phase function Φ(x, y) is expanded to obtain the deformation function Δz(x, y, t).

The above is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Those skilled in the art can easily think of changes or substitutions within the technical scope disclosed in the present invention. , should be covered within the protection scope of the present invention. Therefore, the protection scope of the present invention should be determined by the protection scope of the claims.

Claims (2)

1. A method for measuring object deformation by heterodyne time series speckle interferometry, comprising the following steps: Step 1: Select a He-Ne laser with an output frequency of linearly polarized light at ω; Step 2: The linearly polarized light becomes orthogonal linearly polarized light with two frequencies after passing through the polarization controller; Step 3: the above-mentioned orthogonal linearly polarized light containing two frequencies is spatially filtered and beam expanded through a spatial filter; Step 4: After the beam expansion, the orthogonal linearly polarized light containing two frequencies is transmitted through the ordinary beam splitter, enters the polarization beam splitter prism and is split into two beams; One of the beams of light is reflected by the polarization beam splitter and passes through the incident plane mirror of the analyzer. After being reflected, it passes through the analyzer again, and is reflected at the polarization beam splitter to the ordinary beam splitter as a reference light; After the other beam of light is transmitted through the polarizing beam splitter, it is incident on the object to be measured and scattered, and the scattered light with the deformation information of the object is reflected back to the polarizing beam splitter and passes through it to enter the ordinary beam splitter as the measurement light; Step 5: The measuring light and the reference light are reflected by the ordinary beam splitter, and interfere after passing through the analyzer to form a deformed speckle pattern of the object; the speckle pattern is imaged on a high-speed CCD; Step 6: Perform Fourier transform on the speckle image obtained by the high-speed CCD to obtain a modulation frequency image and a spectrum image, wherein the deformation direction of each part of the object is determined through the modulation frequency image; the time-based deformation of the object is obtained through the frequency spectrum image. 2 . The method for measuring object deformation by heterodyne time-series speckle interferometry according to claim 1 , wherein the polarization controller is a lithium niobate polarization controller. 3 .

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